That sounds right, except that I'm not sure what you mean by "citizen's salary". If you figure that an individual at poverty level spends maybe $7500 a year on taxable items, then the prebate would only be around $200/month.

Having read their material, I'm not sure how they would handle families and/or variances in cost-of-living across of the country. Though, the current federal income tax doesn't recognize cost-of-living differences when calculating its standard deductions, so maybe the FairTax wouldn't need to either.